1. Field of the Invention
The present invention relates to a light-scanning device that scans an object by a light beam, and a display apparatus.
2. Description of the Related Art
Light-scanning devices that scan an object by reflecting light are used in projection type display apparatuses such as a laser projector, a head mount display or a head-up display, laser printers, bar-code readers, and the like. Light-scanning devices using a polygon mirror, a galvano mirror, or the like are known. However, a subminiature light-scanning device (microscanner) that is manufactured of a silicon (Si) wafer by a micro electro mechanical system (MEMS) technique has recently attracted much attention. In such a light-scanning device, torsion springs (torsion beams) is used as a pair of beam portions that support both ends of a mirror unit, and thus the mirror unit vibrates. The light-scanning device is driven at a high speed with a resonance frequency regarding mechanical strength, thereby attaining a necessary scanning angle using a low driving force.
When the light-scanning device is applied to a display apparatus, the light-scanning device is required to be driven with a higher resonance frequency and at a larger scanning angle in order to obtain a high resolution. A spring constant of a torsion spring is required to be increased in order to design the light-scanning device with a higher resonance frequency, but at the same time a material limit of a spring stress is decreased. Thus, in a brittle material such as silicon, a torsion spring is destroyed (or fractured) at a smaller scanning angle. That is, the resonance frequency and the scanning angle have a trade-off relationship, and it is difficult to realize a high-resolution display apparatus such as XGA, SXGA, or HDTV formats, using the previous light-scanning devices.
The inventors of the present invention have suggested a light-scanning device in which two beam portions support both ends of a mirror unit and four arm portions support four beam portions in order to increase the spring constant without increasing the stress subjected to the torsion spring (Patent Document 1). According to the research of the inventors, in such a light-scanning device, a resonance frequency of 38 kHz and a scanning angle of 55° or more are realized in a mirror width of 1 mm. This result may not be obtained in a structure of the related art, and allows the XGA and SXGA resolutions to be realized.
It has been reported that a scanning angle of about 90° is required when a mirror width is 1 mm and a resonance frequency is 36 kHz in order to realize HDTV resolution.
However, even if the light-scanning device disclosed in Patent Document 1 is used, there is a concern that a beam portion may exceed a limiting stress to thereby be destroyed when driving at the scanning angle of 90° required for HDTV resolution or a scanning angle may be insufficient.    (Patent Document 1) Japanese Laid-Open Patent Publication No. 2000-214407